Square to where I am aiming, I set up with a square stance and swing the putter striaght back and striaght through. I haven't ever thought of doing it differently.
The ball will tell you...mostly.... it goes 85% of where your face is pointing(low friction), so if your path is close to straightish, where the ball goes is where your face is pointing.
or
casio...film it.
tomi or puttlab....measure it...